1. Field of the Invention
The present invention relates generally to footwear and more specifically to footwear having a sole which includes an insert that is capable of storing impact energy and returning it to the wearer for increased efficiency.
2. Description of Background Information
A considerable amount of research and development has been carried out in recent years to improve the soles of shoes, especially athletic shoes designed for running and/or jumping. Good impact absorption is an essential property for the soles of running and walking shoes, as well as for shoes used in sports which require jumping, e.g., basketball and aerobics. Not only must a sole absorb shock, but it must be durable to withstand repeated cycles of compression and expansion and it must do so resiliently. The sole must be dense and rigid enough to prevent the wearer's foot from bottoming out upon impact.
Lateral stability of the sole is also important in helping to prevent injuries to the wearer, and therefore the sole must be designed to accommodate vertical movements of compression while resisting lateral and horizontal shear movements. Recently, it has been discovered that a runner or jumper can improve performance by use of an athletic shoe which is more efficient in returning energy to the wearer during the expansion phase of the sole. In this respect, an efficient sole should be capable of storing some of the absorbed impact energy and returning it in a spring-like fashion to assist in propelling the wearer.
Many different approaches have been taken in designing a sole which achieves the above requirements. One type of sole structure which has been designed specifically to attempt to give an appropriate impact response is a sole which contains fluid, either in the form of a liquid or a gas. However, unreliability is often a problem with these types of soles as they may develop leaks.
Another type of sole, which is, e.g., disclosed in U.S. Pat. No. 4,364,188, provides a sole made of a first resilient material having a given durometer hardness for absorbing shock. Plugs having a higher durometer hardness are inserted into the sole to provide an increased resistance to compression.
U.S. Pat. No. 4,342,158 discloses a biomechanically tuned shoe which incorporates a large, bellows-shaped main spring in the heel of the sole of the shoe. The main spring exhibits a large vertical compliance which can impart an unnatural feel to the runner's stride, and disrupt the natural rhythm of the runner's gait.
U.S. Pat. No. 4,815,221 discloses a sole member having a cavity for receiving a spring system which includes a spring plate, a plurality of resilient and compressible projections, and a stiffening member against which the projections exhibit resilient force.
Yet another type of sole is disclosed in U.S. Pat. No. 4,128,950, which discloses a multi-layered sole formed of a closed cell foam, having a semirigid, flat stabilizer plate positioned therein.
Thus, it is desirable to provide a sole with a resilient member which will efficiently store energy and return it to a wearer.